1. Field of the Invention
The present invention refers to an image forming method by a non-contact development using a sleeveless magnet roller as a developer conveying member.
2. Description of the Related Art
For conventional copiers, printers, or facsimile terminal equipment, image forming apparatus using the electrophotography process or the electrostatic recording process is widely known. Generally, according to these processes, a developer is supplied to the developing region between an image-bearing member and a developer conveying member opposed to each other by using the developer conveying member. By causing toner in a developer to deposit to an electrostatic latent image formed on the surface of image-bearing member with an optical image exposure, an image forming is carried out.
The main part of the developer conveying member is a magnet for conveying a developer formed on the roller, on whose surface a sleeve made of non-magnetic materials is provided. The above roller-shaped magnet (hereinafter referred to as a magnet roller) has a plurality of magnetic poles on the surface and a sleeve covers the surface of the magnet in such a manner as to be rotatable to each other.
Also on the surface of the sleeve, a developer thickness regulating member (hereinafter referred to as doctor blade) for regulating the developer layer held on the surface of said sleeve to be constant in thickness is oppositely placed.
The developer is allowed to pass through a small gap (hereinafter referred to as doctor gap: Dg) between the surface of the sleeve and the doctor blade while magnetically adsorbed and held to the surface of the sleeve, so that a thin layer of developer is formed at a predetermined thickness and conveyed to the developing region in which the magnet roller and the image-bearing member are oppositely placed.
In recent years, for this type of image forming apparatus, not only an improvement in image quality but also the low-cost downsizing is strongly required. To meet such a request, various proposals are made for developing apparatus. For example, the development of electrostatic latent images using a magnet roller without a sleeve provided on the surface of the magnet, what is called, a sleeveless magnet roller, has been proposed (e.g. GB2150465A, Japanese Patent Laid-Open Publication No. 223675/1988, and Japanese Patent Laid-Open Publication No. 201463/1987).
To save the production cost, a sleeveless magnet roller is often produced using resin (including elastic materials such as rubber) through injection molding or the like. According to such a sleeveless magnet roller, it is said that no sufficient triboelectrostatic charge of toner in the developer is performed and the image quality is poor.
Accordingly, it is also proposed that a fine electrode for toner attraction is provided on the surface of the sleeveless magnet roller, but to newly install a fine electrode needs labor and so goes against the object of cost-saving and downsizing by using a sleeveless magnetic roller.
Consequently, a proposition is also seen in which electrically conductiveness of the surface of a magnet roller enables a bias to be applied to a developer (Japanese Patent Laid-Open Publication No. 201463/1987). However, this proposition is better in cost than the case of installing a fine electrode, but no sufficient triboelectrostatic charge can yet be accomplished. Thus, there is a problem in that scattering of toner from a magnetic roller rotating at a high speed makes the background fog likely to occur.
On the other hand, various attempts are made also on the opposing relation between an image-bearing member and a magnet roller in the developing region.
In what is called a magnetic brush development in which latent images on the surface of an image-bearing member are rubbed by using a magnetic brush comprising developer layer whose thickness is controlled with a doctor blade, a contact development process having a small gap (developing gap: Ds, e.g., about 0.5 mm) provided between the image-bearing member and the surface of a magnet roller is known. On the other hand, another contact development process is also proposed in which what is called a soft magnetic roller made of softer materials than an image-bearing member is provided in contact with the surface of the image-bearing member.
In the above method, however, because the developing gap is small, developer is likely to deposit to a portion other than the latent image ones and the occurrence of background fog presents a problem. In addition, fluidity is required for developer to be used, for example, it is known that when fluidity reduces due to a change in humidity and other causes, developer sticks or the like fast to the portion of the doctor blade, thus causing a bad effect on the image quality. To solve such problems, still another method called a jumping development, or non-contact development, has been proposed in which a wider gap of developing than the conventional is set and developer is made to jump from the surface of the magnet roller to the surface of the image-bearing member.
On the basis of these various attempts, a non-contact development process using a sleeveless magnet roller low in forming cost is considered from the standpoint of attaining the cost-saving and downsizing and preventing the occurrence of background fog in image quality. In this type of conventional process, however, a problem is pointed out that the insufficient triboelectrostatic charge of developer allows developer to scatter from the surface of the speedily rotating magnet roller, thereby causing the background fog to appear in spite of a noncontact development.